1. Field of the Invention
The present invention relates to a defect determining device for determining a defect of a turbo charger having a boost pressure changing mechanism, which allows change in boost pressure in accordance with the state of operation of an internal combustion engine.
2. Description of the Background Art
A turbo charger utilizing exhaust gas pressure has been widely known as a super-charging system for forced feeding of intake air to the combustion chamber of the internal combustion engine, to attain higher charging efficiency. In the turbo charger, it is desirable to appropriately adjust the boost pressure in accordance with the state of operation of the internal combustion engine. To meet this demand, a turbo charger has been practically used, which includes a variable nozzle provided in a passage of supplying the exhaust gas to the turbine wheel to change the cross-sectional area of the passage, and in which flow and velocity of the exhaust gas blowing into the turbine wheel are adjusted by changing the open position of the variable nozzle.
In the turbo charger having such a boost pressure changing mechanism, a target boost pressure is set in accordance with the state of operation of the engine, and the open position of variable nozzle is changed so that the actual boost pressure matches the target boost pressure. As a result, the actual boost pressure of the turbo charger can be changed to a value suitable for the state of operation of the engine.
In the turbo charger of this type, if the function of changing boost pressure were degraded because of sticking of variable nozzle, for example, it would be difficult to appropriately control the boost pressure in accordance with the state of operation of the engine. Therefore, a degree of deviation between the target boost pressure and the actual boost pressure is monitored, and when the degree of deviation becomes large, it is determined that the boost pressure changing mechanism of the turbo charger is defective, and necessary measures are taken to address the defect. An example of such a device that determines the defect of turbo charger is proposed in Japanese Patent Laying-Open No. 10-196381.
In a turbo charger including a boost pressure changing mechanism, the following defective situation may arise. Specifically, when the variable nozzle is in a closed state with the flow and velocity of exhaust gas blowing into the turbine wheel being large and the nozzle should undesirably be stuck in this state, the boost pressure changing mechanism fails to operate with the degree of supercharging attained by the mechanism kept relatively large. In such a defective situation, the flow and velocity of exhaust gas flowing to the turbine wheel would be unnecessarily large, and the exhaust gas pressure in the exhaust passage from the combustion chamber of the engine to the turbo charger would also increase. Consequently, the increased exhaust gas pressure may cause damage on pipes of the exhaust system, or may undesirably open the exhaust valve, possibly leading to leakage of exhaust gas back to the combustion chamber of the engine. Therefore, it is desirable to determine and address such a defective situation as soon as possible.
The determination of any defect of the turbo charger based simply on the degree of deviation between the target and actual boost pressures, however, may result in the following problem. Specifically, assume that the target boost pressure changes significantly along with the change in the state of operation of the engine. In such a situation, the actual boost pressure may not follow the change in target boost pressure fast enough, due to delayed response, for example, of the boost pressure changing mechanism. In that case, though the boost pressure changing mechanism is not defective and the actual boost pressure would eventually converge to the target pressure, the temporal deviation leads to an erroneous determination that the mechanism is defective. In order to avoid such an erroneous determination, it may be possible to determine any defect if the state where there is a deviation between the actual and target pressures continues for a prescribed time period. It is noted, however, that the time necessary for the actual boost pressure to converge to the target pressure differs dependent on the amount of change in the target boost pressure or aging degradation that may affect response of the boost pressure changing mechanism. Therefore, in this method, it is necessary to set the prescribed time period a little longer, expecting such difference.